已知R2-离子的核内有n个中子，R的质量数为M，则mgR2-离子里含有电子的物质的量为（　  ）mol

A.mol                          B.mol

C.mol                    D.mol

A.mol                                 B. mol

C. mol                           D. mol

A. mol                             B. mol

C. mol                       D. mol

A. (A－N+8) mol                    B.(A－N+10) mol

C.(A－N+2)mol                                  D.(A－N+6) mol